README.md: add badge to pkg.go.dev

Change-Id: Iae82dbce741d2eaa03bcae046842589f0764c933
Reviewed-on: https://go-review.googlesource.com/c/text/+/275890
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Trust: Julie Qiu <julie@golang.org>

README.md

@@ -1,5 +1,7 @@
 # Go Text
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/text.svg)](https://pkg.go.dev/golang.org/x/text)
+
 This repository holds supplementary Go libraries for text processing, many involving Unicode.
 
 ## Semantic Versioning